\section{What can not happen}
Three situations are clear that should not happen:

\begin{itemize}
	\item If the input is not valid, the output cannot be valid neither create an exception.
	\item If there is an exception at the fetch level that arrives to the decoder (being a valid fetch), then the output of the decoder cannot change. This latter means that the exception originated at the fetch, remains and the instruction is not decoded.
	\item If the input is valid and has an illegal exception, the output should be an exception of illegal instruction.
	
\end{itemize}

    